In general, in a transmission/reception system using, in particular, transmission/reception as one antenna, in TDD (Time Division Duplexing) communications having the same transmission/reception frequency such as Wibro or RF, it is indispensably necessary to design a circuit unit with a high degree of isolation between transmit and receiving parts so as to prevent the receiving part from being broken due to a high-power transmission signal. This circuit unit is generally configured using a circulator or an RF switch. The circulator is advantageous in a high-power, but is disadvantageous in that it has a low degree of isolation and is bulky and expensive. An integrated chip type RF switch is advantageous in that it has a wide bandwidth and a small volume, but is disadvantageous in that it has a low output and a low degree of isolation. A ring type RF switch is advantageous in that 1) it can be fabricated easily and has 2) a high-power, 3) a high degree of isolation, and 4) a low insertion loss through attenuation by a 180 degree phase difference, but is disadvantageous in that it has a narrow bandwidth and is difficult to miniaturize in a low frequency band since it has a size proportional to a wavelength length of a design frequency.
The conventional technology and the problems of the conventional technology are described below with reference to the drawings.
FIG. 1 is a view illustrating a general ring switch.
Referring to FIG. 1, a general ring switch 10 includes a 3λ/4 RH transmission line (−270 degrees) 11, a λ/4 transmission line (−90 degrees) 12 and PIN diodes 13. The PIN diode 13 is an element, which has an excellent linearity and very small distortion and can be switched at high speed, and is equalized as shown in FIG. 2. In FIG. 2, Lp and Cp denote the inductance and capacitance by a package, Ci denotes intrinsic layer capacitance, Rs denotes a serial resistor, and Ri denotes a variable resistor by a control current.
When a control current is applied to the ring switch 10, the PIN diodes 13 become short. Signals applied through a terminal 1 have a 180 degree phase difference through the 3λ/4 RH transmission line 11 and the λ/4 transmission line (−90 degrees) 12, so that they are attenuated in a terminal 2. Further, a signal reflected from the terminal 2 due to mismatching is also attenuated in the terminal 1. Thus, the ring switch 10 operates as a switch when the control current is applied thereto.
The ring switch employing this RH transmission line and the PIN diode can be fabricated and designed easily, but is problematic in a narrow bandwidth.
To solve the problem, active research has recently been done into MM (Meta-material) having a negative dielectric constant and conductivity and microwave elements employing the MM have been developed. Research on MM or LHM (Left-Handed Material) was first begun by Velselago who was a Russian physicist in 1967. The MM or LHM has a negative dielectric constant and transmittance and therefore shows peculiar electromagnetic characteristics, such as phase and group velocity with opposite directions and a negative reflection coefficient. The electromagnetic characteristics of the LHM can be implemented through an artificial structure and a structure of the LHM is composed of a unit cell. The cell must have an electrical size, which is ¼ or less of a guided wavelength. This is called an effective-homogeneity condition.
An application of the LHM to microwave elements is implemented through a combination of serial capacitance and parallel inductance when a general transmission line is equalized based on a lossless transmission line mode. However, an ideal transmission line of the LHM cannot be implemented due to the loss of the current and voltage according to electric waves, and therefore can be equalized as a CRLH transmission line in which the RH characteristic is incorporated. If this CRLH transmission line is applied to microwave elements, it can be applied to broad-band, miniaturized and dual band designs.